[WIP] Audit logs in JSON

See https://gist.github.com/manasdk/50cf0f97c069da131274 for a sample of an audit log file generated by st2actions.

- Audit logs in JSON so that they can be easily parsed by logstash, splunk etc.
- Using https://github.com/madzak/python-json-logger as the log formatter. May write own JSONFormatter or contribute to python-json-logger if they are amicable with my choices.

## TODO

- [ ] Fix regular logs where the extra fields are ignored. - Manas
Yep.

And all the logger methods from python logging module already take `extra` argument (see https://docs.python.org/2/library/logging.html#logging.Logger.debug) which makes wrapper unnecessary.